1. Field of the Invention
The present invention relates to an optical waveguide device such as a variable optical attenuator or optical switch that is used in the technical field of optical signal transmission and that uses the phase shift of an optical signal. More particularly, the present invention relates to an optical waveguide device that uses the thermo-optical effect, and to a variable optical attenuator array or optical switch array that is configured by arranging a plurality of these waveguide devices on the same substrate.
2. Description of the Related Art
With the expansion of the amount of information transfer that passes over telecommunication networks, WDM (Wavelength Division Multiplexing) transmission systems that dramatically increase the transmission capacity of optical fiber circuits have come into wide use. In a WDM transmission system, since signal light of a plurality of wavelengths is transmitted over a single optical fiber, light amplifiers are therefore widely used for compensating for optical loss or for boosting optical power. EDFAs (Erbium Doped Fiber Amplifiers) are the most widely used light amplifiers.
An EDFA is a direct light amplifier and uses optical fiber in which erbium (Er) has been added to the core of the optical fiber, and although such amplifiers have the characteristic of large light amplification gain, their amplification characteristics are also strongly dependent on wavelength. When the optical transmission path is designed with an EDFA having large wavelength dependency, the optical power level that is applied to the optical fiber will vary widely between channels that each have a different wavelength, and deterioration in the optical waveform may therefore occur due to such factors as nonlinearity in a portion of the channels. Various measures have been introduced for compensating for this wavelength dependency of EDFA.
As one such measure, a method has been adopted in which a loss medium for compensating for fluctuation in the gain of an EDFA is introduced in the optical transmission path, whereby the light amplification gain of EDFA is made uniform. Examples of such a loss medium that can be used include a long-period fiber Bragg grating in which a portion of the optical fiber is subjected to optical processing or an optical component in which a multiplicity of FP etalons are combined. The overall wavelength dependency of gain can thus be reduced by increasing the loss due to the loss medium in wavelength regions having large EDFA gain.
Nevertheless, gain adjustment that employs a loss medium has limits when the wavelength region extends over a wide bandwidth. In one method in actual use, optical attenuators are inserted for each channel of a WDM transmission system and the optical power is then fine-adjusted for each channel. A Mach-Zehnder type variable optical attenuator (VOA) is used for this purpose.
FIGS. 1A and 1B show the configuration of a Mach-Zehnder variable optical attenuator of the prior art. Variable optical attenuator 100 shown in the figure employs substrate 101 in the form of a flat sheet made of silica glass on which planar lightwave circuits (PLCs) are constructed. Glass waveguide 102 is embedded in the interior of substrate 101, and in the center portion of substrate 101, glass waveguide 102 is branched into two waveguide sections 102A and 102B. In other areas, incident-side waveguide section 102C and emission-side waveguide section 102D are formed. In other words, light that is incident from waveguide section 102C is branched into two beams by the Y-branch to waveguide sections 102A and 102B, and then combined by another Y-branch to be emitted to the outside from waveguide section 102D.
Heater 109 is formed on the surface of substrate 101 at a position that is above waveguide section 102A. First and second electrodes 107 and 108 are formed on the surface of substrate 101 for supplying electric power to heater 109, and heater 109 is electrically connected between electrodes 107 and 108. The application of a prescribed voltage from a power supply circuit (not shown) to first and second electrodes 107 and 108 causes-heater 109 to turn ON and give off heat, whereby waveguide section 102A that corresponds to this heater 109 is heated. Generally, metals, which can be incorporated in the processes during formation of the waveguides, such as gold (Au), platinum (Pt), and chromium (Cr) are used in first and second electrodes 107 and 108 and heater 109. These metals feature low specific resistance, no deterioration, extremely stable characteristics (particularly for gold and platinum), and are amenable to evaporation.
In this configuration, the two waveguide sections 102A and 102B have the same length, and the temperature of the two waveguide sections 102A and 102B is equal when heater 109 does not emit heat. In this state, light that is propagated in waveguide section 102C on the input side is branched by two waveguide sections 102A and 102B and then combined at waveguide section 102D on the output side in the same phase state. Light that is combined at waveguide section 102D on the output side therefore has no phase difference and no loss occurs.
In contrast, when the flow of current in heater 109 causes heating of one waveguide section 102A, the refractive index of that waveguide section 102A increases. As a result, of the light that is branched between the two waveguide sections 102A and 102B, the phase of light that is propagated through waveguide section 102A that has been heated is gradually shifted and delayed depending on the increase in temperature. Therefore, a phase difference is produced between the light that is propagated through one waveguide section 102A and light that is propagated through the other waveguide section 102B. The light that is supplied from waveguide section 102D on the output side is attenuated as this phase difference approaches 180 degrees (i.e., the π radian). The maximum attenuation of the light output is reached when the phases of the light that is propagated through the two waveguide sections 102A and 102B differ by 180 degrees. When the phase of the light that is propagated through waveguide section 102A is further shifted, the phases of the two beams approach the same phase, and the light that is supplied from waveguide section 102D on the output side increases.
Japanese Patent Laid-Open Publication No. 10-20348 (JP, 10-020348A) discloses a thermo-optic phase shifter and optical attenuator that employ a polymer optical waveguide and that operate based on the principles described above.
However, it is known that the characteristics of waveguide devices that use the thermo-optic effect, such as optical phase shifters, optical switches, and optical attenuators, change with the ambient temperature. For example, if a fixed voltage is applied to a heater of a waveguide device, the level of light attenuation will vary according to the ambient temperature. The nature of this change in the characteristics will vary depending on the materials used to construct the waveguide device or the configuration of the waveguide device. For example, a measurement of the temperature dependency of the extinction characteristics in the variable optical attenuator shown in FIGS. 1A and 1B produces the measurement results that are shown in FIG. 2. Here, Vπ is the voltage at which the extinction ratio reaches a maximum, this being the ratio of the maximum intensity to the minimum intensity of transmitted light in variable optical attenuator 100. This voltage Vπ tends to increase together with increase in temperature T. Even if variable optical attenuator 100 is set to a prescribed amount of light loss, this amount of loss will fluctuate with fluctuations in ambient temperature, with the resulting problem that the amount of loss of light cannot be set with high accuracy.
Various investigations have been conducted regarding the reasons for this change in the amount of loss of light due to temperature. In Japanese Patent Laid-Open Publication No. 6-34924 (JP, 6-034924A), the change in the temperature of the resistance of the heater that heats one waveguide section is taken as the source of the problem. In an optical phase shifter and optical switch which are based on the thermo-optical effect and which employ silica glass optical waveguides, and further, use directional couplers in the waveguide branches, the invention of JP 6-434924A therefore specifies the range of the temperature coefficient of resistance of the material that makes up a thin-film heater which heats one waveguide section in the optical phase shifter and optical switch. However, despite the reduction of the temperature coefficient of resistance of the material that makes up the thin-film heater, the temperature dependency remains to a significant degree.
For preventing fluctuations in the amount of light loss that are caused by temperature fluctuations, it has also been proposed that a waveguide device such as a variable optical attenuator be provided with a temperature control circuit for reducing temperature fluctuation of the device itself. However, the provision of a temperature control circuit introduces the problems of the resulting complexity of the circuit configuration of the waveguide device and further, the increase in product cost for realizing a highly accurate circuit.
Japanese Patent Laid-Open Publication No. 5-150275 (JP, 5-150275A) discloses the arrangement of a device in which an assembly of normal resistance and thermistor connected in series is connected to a thin-film heater in parallel as a temperature compensation circuit in an optical switch that is based on the thermo-optical effect and that uses silica glass optical waveguides and directional couplers in the waveguide branches. This temperature compensation circuit takes into consideration the temperature dependency of thermal conductivity in a waveguide device and compensates for the temperature dependency of the waveguide device by controlling the electric power that is applied to the heater.
The foregoing explanation regarding changes in attenuation characteristics that accompany fluctuations in the ambient temperature assumed the case of a single variable optical attenuator, but the problems are still more complex in an array device in which variable optical attenuators of the same configuration are formed on the same substrate as a multiple-channel configuration. In an arrayed waveguide device that is formed with this type of array configuration, causing a heater to emit heat to adjust the corresponding optical attenuator that is provided for a particular channel heats not only the corresponding part of the substrate, but also conveys heat to its vicinity as well. This causes fluctuation in the amount of loss of light of similar variable optical attenuators, particularly variable optical attenuators in adjacent channels, giving rise to the phenomenon of thermal crosstalk.
To avoid the occurrence of thermal crosstalk, a configuration has been adopted in which a heater is separately incorporated inside the module for temperature control. However, a module of this type of construction necessitates not only the incorporation of a heater but also the implementation of temperature control for the variable optical attenuator of each channel, raising not only the problem of increased module price, but also the problem of wasted power for temperature control. In other words, the problems attending the provision of a temperature control circuit in a single variable optical attenuator become even more prominent.